In recent years, environmental pollution caused by nitrogen oxides (NO.sub.x gases) and particulate materials (PM) which are discharged from internal combustion engines, particularly Diesel engines, is an important problem, and decreasing nitrogen oxides and particulate materials in exhaust gas is urgently required.
For overcoming the problem, exhaust gas recirculation (EGR) and lowering of the peak temperature of combustion by delaying time of fuel injection are examined for decreasing NO.sub.x gases.
However, when the peak temperature of combustion is lowered, black smoke and PM are increased, and attaching an apparatus for exhaust gas treatment becomes necessary. As the apparatus for exhaust gas treatment, PM traps or oxidation catalysts are examined. Because these apparatuses have a structure of a filter, clogging by metal components in oil causes a problem when conventional Diesel engine oil is used.
In lubricating oil compositions for internal combustion engines, an ashless dispersant and a metallic detergent are generally contained as the essential components. However, decrease in metallic components is desired because of the problem of clogging, as described above. This causes another problem in that excellent effects of keeping an engine clean and preventing formation of deposit become difficult to be obtained when metallic detergent is not used or used in a small amount.